In the related art, an EGR device that forces a part of an exhaust gas to flow back to the intake system of an engine and reduces generation of a nitrogen oxide has been developed as a heat exchanger. The EGR device is provided with an EGR cooler for cooling the exhaust gas, and the EGR cooler is mounted between the exhaust and intake systems of the engine of a vehicle.
As the EGR cooler, a plate tube type cooler has numerous flat plate tubes that are inserted into and installed in a shell formed as a cylinder, and performs heat exchange between an exhaust gas flowing inside the tubes and a coolant flowing outside the tubes.
Each of the tubes is used as a tube in which an inner fin is inserted into a flat tube main body that is extruded and is molded in a hollow shape by rolling or the like, or that is formed of members divided into two upper and lower parts, and the tube main body and the inner fin are brazed.
In the related art, an inner fin for a heat exchanger is disclosed in, for instance, Patent Document 1. The inner fin is an inner fin which is mounted in a flat tube, in which a thin sheet forming the inner fin is formed in a serpentine shape in which each curved crest portion is alternately in contact with both inner walls of the flat tube facing each other, and which has a first corrugated shape in which an exhaust gas channel is formed between partitions and a second corrugated shape in which a wall of an accordion structure is formed. The inner fin is a so-called wavy fin, and is an inner fin that more hardly filled with soot than an offset fin.
A heat exchanger in which numerous fins are fixed inside a flat tube and numerous V-shaped strip portions at which a cross section of the fin in a circulating direction of a gas has a corrugated shape are bent is described in Patent Document 2. The heat exchanger is a heat exchanger in which one of a pair of oblique strip portions forming a V shape is inclined at an angle of α and is disposed on a positive side and the other is inclined at an angle of β and is disposed on a negative side, and in which both the oblique strip portions are disposed at an asymmetrical angle. Thereby, large and small vortex flows are formed on a diagonal line inside a segment of each fin, and particulate matter of a trough portion or the like of the fin is effectively blown away.
An exhaust inner fin for a heat exchanger represented in Patent Document 3 is an offset fin in which, when viewed from a flow direction of an exhaust gas, a corrugated portion caused by a cut and raised portion is offset with respect to the neighboring corrugated portion. The exhaust inner fin has a shape in which a wall portion dividing an interior of a tube into a plurality of channels is disposed in a zigzag shape in the flow direction of the exhaust gas, and convex portions adjacent to each other in an exhaust flow direction are disposed out of alignment.
A fin for a heat exchanger represented in Patent Document 4 divides an exhaust passage into a plurality of segments that repeat a rugged shape in an exhaust flow direction and a direction perpendicular to a tube stacking direction and are formed in an offset shape in which they are alternately shifted in the exhaust flow direction at each predetermined length. A horizontal wall constituting each of the segments is formed by cutting and raising a plurality of projected plate.